This invention relates to an improvement on the invention described in U.S. patent application, Ser. No. 909,741 filed in the name of Robert B. Crowell on May 25, 1978, now Pat. No. 4,178,832. The present invention is especially concerned with means for removing powder-like combustion residues from the sliding surfaces in the mechanism disclosed in the already-filed patent application. The powder-removing mechanism includes a system of radial grooves in the piston-cylinder end wall interface, and at least one annular pocket in the cylinder side surface. Reciprocal motion of the piston causes the residue particulates to collect in the annular pockets where they cannot prematurely wear the sliding surfaces or jam the components against normal movement.